The use of electret ionization chambers for detection of radioactive gases, for example, radon gas, has recently gained general acceptance as an accurate technique for measurement. An electret is a non-conductive material, for example, Teflon.TM., that is capable of holding an electrostatic charge of either polarity for a long period of time. The electret attracts charged particles of opposite polarity relative to the surface charge given to the electret. Each charged particle that contacts the electret surface in the ionization chamber-neutralizes a portion of the surface charge. By measuring the change in apparent electrical charge on the electret after a known period of exposure, it is possible to compute the concentration of charged particulates and radioactivity in the ambient environment.
Because exposure to radon gas presents a health threat to individuals, detection of radon gas, especially in the home, in schools and in work places, has become a problem warranting attention. Estimates of the number of homes in the United States that should be tested for high concentrations of radon, range in the millions. Testing, to date, represents a considerable expense to many homeowners because the detectors which must be placed in the home for radon detection, and the laboratory equipment for quantifying radon concentrations after measurement has been completed, are expensive per se and tests are usually performed by trained personnel.
Manufacturers are attempting to provide do-it-yourself detectors that the homeowner, for example, can position within his own home for measurement purposes. After exposure, the detectors are sealed and returned to the manufacturer for evaluation of the radon concentration levels that exist at the test site. Results and recommendations are duly reported to the user. However, even with simplified devices, costs can still be prohibitive, for example, in evaluating radon levels in public schools, where several hundred detector units may be required in a single building.
Problems have been inherent in electret detector systems whereby inaccuracies in measurement result from improper handling of the devices both by the manufacturer and the end user. In particular, any contact with the charged surface of the electret, for example, by a person's fingers, or dust particles in the air, neutralizes a portion of charge on the electret surface and creates inaccuracies in measurement. This problem is being overcome in marketed products by providing completely assembled detector units to the user who only has to unseal and seal the unit and record dates during the test period. The detector unit is then returned to the manufacturer for evaluation of radon concentration levels.
However, other factors that are beyond the control of the end user of such commercial products exist which affect the accuracy of the test readings. One major factor is background radioactivity which affects the charge on the electret. For example, accuracy is reduced by the presence of environmental gamma rays in the atmosphere, especially of terrestrial origin such as from the uranium family present in certain rocks including granite. Terrestrial sources of gamma rays vary from region to region due primarily to the geological composition of that area. Gamma rays penetrate the ionization chamber, even when the chamber is sealed against the entry of radon gas and charged particles. The gamma rays penetrating the chamber walls result in a reduction in the charge on the electret. Thus, in evaluating test results when measuring radon, it has been necessary to eliminate the influence of gamma rays on the data, primarily by the use of correction factors based upon theoretical and empirical calculations. Thus, the effect of the presence of gamma rays on the accuracy of the radon measurement is based to some degree upon approximations.
A typical short term electret measurement made by exposure to the atmosphere over a 2 to 7 day period is sufficient in most cases to determine if there is a radon problem. However, it is known that there is a seasonal variation in radon levels. As a result it is desirable to employ long term detectors when marginal levels are detected by short term tests. These long term measurement detectors use electret chambers designed to decay at a slower rate than short term electret chambers. As a result an average radon concentration over several months may be detected.
What is needed is a radioactivity detector system that is economical to produce, is usable by untrained personnel without loss in measurement accuracy, discriminates radon levels from background radiation produced by the presence of gamma rays and can be adapted for both long term and short term measurements.